BERLIN (Reuters)—German officials hunting the source of an E. coli outbreak said they were investigating a caterer that served some of the victims, as the death toll from the bacteria climbed to 30.

German Chancellor Angela Merkel said authorities were putting "maximum" effort into tackling the outbreak, in an apparent bid to deflect growing international and domestic criticism over the government's handling of the situation.

The outbreak has infected more than 2,900 people in 12 countries. All cases have been traced back to the Hamburg area in northern Germany.

Authorities in the central Kassel region said they were investigating a caterer that delivered food to a birthday party near the town of Goettigen, Germany, on May 28 where at least eight people fell ill.

At least two of the sick people had tested positive for E. coli and one for the more dangerous complication called hemolytic uremic syndrome (HUS), said the authorities.

"At the moment tests are under way... Clearly the (catering) firm was shocked, but they are cooperating with us in a very constructive manner," said Markus Schimmelpfennig, deputy director of the Kassel region's health authority.

Authorities in Frankfurt, Germany, on Thursday said a man who had traveled to Hamburg in late May had died in a city hospital, bringing the death toll from the outbreak to 30.

'Complicated operation'

"It is still a very complicated operation, finding the source, but I feel there is great agreement to work on it with maximum effort," Chancellor Merkel said at a news conference.

Authorities continue to focus on German-grown bean sprouts in the investigation, although cucumbers, once at the center of suspicion, returned to the spotlight briefly after traces of the E. coli strain were found on the vegetables in Saxony-Anhalt.

Analysis of samples from restaurants, canteens and kitchens that prepared food eaten by patients has so far failed to yield conclusive evidence for the leading theory that organic sprouts from a farm in Lower Saxony state are behind the outbreak.

Late on Thursday, German media reported that the farm, which has been investigated by experts in white lab coats for several days and is banned from selling the sprouts, can still legally sell other vegetables.

A spokeswoman for the Lower Saxony consumer protection agency told Der Tagesspiegel daily the issue was being discussed, while website Spiegel Online said authorities believed tomatoes and parsley had been sold after the farm was closed.

"At the moment it is being examined whether the company can be legally prohibited from selling (other) vegetables," the spokeswoman told Tagesspiegel.